The present invention is generally directed to optical fiber assemblies, and more particularly to an optical fiber assembly which allows sufficient curing of an interiorly positioned epoxy to provide optical stability to the optical fiber.
Several different techniques are known for attaching optical fibers to optoelectronic packages. Two of the more widely used techniques are solder attachment of a metallized optical fiber in a metal sleeve mounted to a wall of an optoelectronic package, and laser welding of an optical fiber assembly to a wall of an optoelectronic package.
When optical fibers are, for example, laser welded to optoelectronic packages, the alignment and positioning of the fiber relative to the optical axis of the optoelectronic package is performed by way of active alignment. Specifically, the optoelectronic package is held in a fixture that provides mechanical stability, spatial positioning, spatial manipulation and electrical biasing for the optoelectronic elements within the package. The optoelectronic elements generally include at least an optical element, such as, for example, a laser diode, photodiode, or lens, an optical fiber assembly including a metal ferrule at one end to be attached to the package and a bare fiber or optical connector, a light source, and a detector. The light source may be a solid state laser inside of the optoelectronic package or a laser source connected to the connector end of the fiber. The detector may be the photodioode or a detector at the connector end of the fiber.
The optical fiber assembly is held in the fixture, and the connector is connected either to a photodetector, in the case of a laser within the package, or to a laser in the case of a photodiode within the package. An electrical bias is then applied to the optoelectronic elements within the package. While the bias is applied, the package and/or the optical fiber is spatially manipulated to find a position which provides a desired level of optical power to the detector. Once the desired level is obtained, the optical fiber and the ferrule are affixed to the optoelectronic package by laser welding.
Typically, laser welding utilizes a high power laser source, such as a YAG laser, and the laser source is positioned to direct light onto the fiber and the portion of the ferrule in contact with the optoelectronic package. When the YAG laser is modulated, the metallic ferrule absorbs the energy locally, causing the temperature of the ferrule and the package to rapidly rise and eventually causing the ferrule and the package to melt locally such that a weld joint between the optoelectronic package wall and the ferrule is formed.
FIGS. 1-2 illustrate an optical fiber assembly 10, which includes an optical fiber 12 positioned within a ferrule 20. The ferrule 20 includes a metallized body 22 with a thin wall section 26 surrounding a defined interior space 24 and a thick wall section 28 surrounding a channel 30. A mid-section of the ferrule body 22, shown between a pair of dashed lines in FIG. 2 and designated generally as element 36, is located between a ferrule body first end 32 and a ferrule body second end 34. The channel 30 leads from the defined interior space 24 to the body second end 34.
The fiber 12 extends through the ferrule 20. The end of the fiber 12 nearest the body first end 32 extends through a channel 16 of a jacket 14. An epoxy 40 fills out the space remaining in the defined interior space 24 and the channel 30 after positioning of the fiber 12 and the jacket 14. The spacing between the fiber 12 and the wall of the channel 30, as well as the spacing between the jacket 14 and the inner wall of the ferrule body 22 is small, typically on the order of a few microns. Such small spacing minimizes the potential for movement of the optical fiber 12. The spacing between the fiber 12 outside of the jacket 14 but within the defined interior space 24 and the inner wall of the ferrule body 22 is significantly larger than the spacing between either the fiber 12 and the wall of the channel 30 or between the jacket 14 and the inner wall of the ferrule body 22. Although an epoxy is shown in the defined interior space 24 in FIG. 2, other materials, such as, for example, a ceramic or other hard material insert may also be placed within the defined interior space 24.
The epoxy 40 is utilized to attach the fiber 12 to the wall of the channel 30 and to the inner wall of the ferrule body 22. The epoxy 40 is inserted within the ferrule 20 in a liquid or semi-liquid form, and during the epoxy cure cycle, the epoxy within the channel 30 and between the jacket 14 and the inner wall of the ferrule body 22 typically cures faster than the epoxy 40 residing in the remainder of the defined interior space 24. The variable curing time is due to a lesser volume in and a more efficient heat transfer through the small open-spaced regions, namely within the channel 30 and between the jacket 14 and the inner wall of the ferrule body 22, than in the large open-spaced region, namely the remainder of the defined interior space 24.
Because the epoxy 40 in the small open-spaced regions cures faster than the epoxy 40 in the large open spaced region and because the small open-spaced regions are on either side of the large open-spaced region, the curing time in the large open-spaced region is further retarded. Curing of the epoxy 40 leads to the production of gaseous reaction products in accordance with Equation 1 below: 
The nomenclature [m], [n], [o] and [p] are constants for balancing out Equation 1. Diffusion of the gaseous reaction products is slower through cured epoxy than through non-cured epoxy. The slower diffusion rates of the epoxy 40 in the small open-spaced regions leads to a build up of gaseous reaction products in the large open spaced region.
The build up of the gaseous reaction products further retards the curing time of the epoxy 40 in the large open spaced region. As illustrated in the equation above, epoxy rings react with a curing agent when energy, such as heat or light, is applied to the system to form a polymeric epoxy material plus the gaseous reaction products. In an equilibrium, Equation 1 above is constant. In other words, as noted in Equation 2 below:                     Constant        =                                                                                                  [                                                                                            Polymeric                                                                                                                      epoxy                                                                                      ]                                    o                                ⁡                                  [                                                                                    gaseous                                                                                                            reaction                                                                                                            products                                                                              ]                                            p                                                                                            [                                                                                            Epoxy                                                                                                                      ring                                                                                      ]                                    n                                ⁡                                  [                                                                                    Cure                                                                                                            agent                                                                              ]                                            m                                =                                    k              1                                      k              2                                                          Equation        ⁢                  xe2x80x83                ⁢        2            
Since the total gaseous reaction products are composed of the gaseous reaction products from new reactions and the trapped gaseous reaction products, and given that the epoxy 40 within the ferrule 20 is an equilibrium system, then as the amount of trapped gaseous reaction products increases, the reaction rate defining the formation of polymeric epoxy decreases. Thus, the ferrule assembly 10 has the potential for creating a system in which the epoxy 40 in the large open spaced region, and especially in the mid-section 36, never fully cures.
The state of cure of epoxy can be defined with reference to its glass transition temperature Tg, which may be defined as the temperature range at which the mechanical properties of a material, in this case epoxy, change such that above the range the epoxy is elastic and below that range the epoxy is brittle. FIG. 3 illustrates the general elastic modulus behavior for a defined epoxy system over a temperature range. The same starting epoxy system, cured to a different degree, namely to a different Tg, is shown in the graph. As shown in FIG. 3, the general elastic modulus behavior of an epoxy system increases with an increasing Tg, while the magnitude of the temperature range over which there is a significant change in the general elastic modulus behavior increases with decreasing Tg.
The change in general elastic modulus behavior with a change in Tg translates into a change in the magnitude and time dependence of the reversible strain which can occur when a stress is applied to the epoxy at a given temperature, as illustrated in FIG. 4, which plots the reversible strain of the epoxy systems of FIG. 3 over time for a given applied stress. The time is given in arbitrary units arbs. The temperature is held constant near the temperature where the onset of the change in the general elastic modulus behavior for the lowest Tg cured epoxy shown in FIG. 3 occurs. The strain is defined as the length of an epoxy under non-permanent, or reversible, strain (X2) minus the length of the epoxy under no strain (X1) divided by X1. As FIG. 4 indicates, a lower Tg leads to a larger reversible strain with longer time constants than a higher Tg at a given stress and temperature.
For optoelectronic packages that use laser welding to attach a fiber ferrule, such as the fiber ferrule 20 shown in FIGS. 1-2, the optical stability at any given temperature is influenced to a large degree by the reversible strain behavior of the epoxy 40. Generally, the larger the reversible strain of the epoxy 40 the greater the potential for shifts in the optical power transmitted or received from the optical package due to changes in the optical alignment. In addition, the longer the time dependence the more probable it will be that the change in optical power will occur after the optoelectronic package has passed various manufacturing screens and is in a third party""s operating system. Such an optoelectronic package is likely to fail in the field.
FIG. 5 illustrates an optoelectronic package operated under system conditions for approximately 170 hours. Starting from time equals zero, the power output begins to degrade, and eventually degrades by about 1.2 dB. The power output degradation can be traced back to an improperly cured epoxy in the large open spaced region of the ferrule 20.
There exists a need to provide a ferrule which compensates for the variable curing time of epoxy, and thereby reduces the number of optoelectronic packages which fail in the field due to improperly cured epoxy induced power output degradation.
The invention provides optical fiber assembly that includes a ferrule having a ferrule body and a fiber extending through said ferrule body. The ferrule body extends between first and second ends and defines a defined interior space. One or more openings extend through the ferrule body to the defined interior space.
The invention further provides an optoelectronic package that includes an optical subassembly and an optical fiber assembly attached to the optical subassembly. The optical fiber assembly includes a ferrule having a ferrule body and a fiber extending through said ferrule body. The ferrule body extends between first and second ends and defines a defined interior space. One or more openings extend through the ferrule body to the defined interior space.
The invention further provides a method for suppressing optical instabilities in an optoelectronic package. The method includes creating one or more openings in a ferrule, inserting a fiber in the ferrule, injecting an epoxy in the ferrule, curing the ferrule, and attaching the ferrule to an optical subassembly.
The foregoing and other advantages and features of the invention will be more readily understood from the following detailed description of the invention, which is provided in connection with the accompanying drawings.